The basic function of a photorefractive device is to collect and analyze ocular responses to light stimuli. Light from an external source enters the eye through the pupil and is focused to create a small illuminated spot on the retina. Some of the light from this retinal spot is returned out of the eye through the pupil after interaction with different layers of the retina. The pattern of light exiting the pupil is determined by the optics of the eye and is dominated by an examinee's refractive error (focusing errors of the eye).
Unlike fundus photography, wherein a large area of the retina is illuminated and a camera is focused on the retina to image details of its anatomy, photorefraction does not directly image the retina or any other structures in the posterior segment of the eye. In photorefraction, images are obtained by focusing on the pupil to obtain the light pattern exiting the pupil—i.e., images are analyzed in the pupil plane.
In earlier known methods of photorefraction, typically only eccentric illumination (i.e., lights arranged outside a lens aperture of an ocular screening system) is used. This approach has limitations and can often result in refractive error determinations that are inaccurate or ambiguous, particularly since eyes with different refractive errors can have similar responses under a given illumination. Classic photorefraction using eccentric illumination alone generates a “crescent-like” reflex in the pupil plane, the edges and domains of which must be determined for purposes of correlating the pupil response with a refractive error. When using eccentric or decentered illumination alone, determination of the crescent boundary is a difficult task. In addition, the determination of pupil size and location is often compromised by not having sufficient pupil edge data (due to dark edges) for accurate pupil circle fitting.
Accordingly, there exists a need to provide improved methods of conducting photorefraction-based ocular examinations.